1. Field of the Invention
The present invention relates to an apparatus for and a method of driving an ink jet record head wherein ink droplets of different sizes may be spouted from each nozzle, and also to a printing apparatus with such a driving apparatus.
2. Discussion of the Background
An ink jet printer converts an image, which is to be printed, into binary codes or multi-value codes and controls the creation of dots on a recording medium by respective nozzles on a print head, based on the dot on-off signals obtained as the results of the multi-value coding. The structure of the ink jet printer causes the plurality of nozzles on the record head to respectively spout ink droplets at a predetermined timing and thereby form dots on the surface of a recording medium, such as a recording sheet of paper, so as to implement the recording. The fundamental technique for ink spouting pressurizes ink in an ink conduit, which is connected to each nozzle, for an extremely short time period and causes the pressurized ink to spout as an ink droplet from the end of the nozzle. There are several known mechanisms for generating the pressure that is applied to the ink; for example, a system that utilizes piezoelectric elements to generate the pressure and a system that takes advantage of bubbles produced by heating to apply the pressure. No matter which mechanism is applied, however, the ink jet technique can only control the spout or non-spout of ink droplets from the ends of the respective nozzles, that is, the on-off state of the dots. It is thus extremely difficult to continuously and freely regulate the weight of ink in each spouted ink droplet and record the halftone.
Several techniques have been proposed to express the halftone; for example, the area tone method, the dither method, and the error diffusion method. The area tone method, for example, expresses each pixel with a plurality of dots like 4.times.4 or 8.times.8 to record the halftone. The expression of one pixel with the 4.times.4 dot matrix results in the representation of the density by the gradation of 16 tones (or 17 tones including complete white). An increase in resolving power of the pixel ensures a more minute tone expression. An increase in tone without varying the diameter of the recorded dots, however, lowers the practical resolution. The larger diameter of the dots recorded on the recording paper makes the granularity undesirably conspicuous in a low density area. It is accordingly necessary to decrease the weight of each ink droplet and reduce the diameter of the recorded dots.
A known technique for spouting ink droplets of a smaller weight to reduce the dot diameter first expands and then contracts the volume of a pressure chamber connected to the ink conduit, which is generally referred to as the `drawing shot`, as described in, for example, JAPANESE PATENT LAID-OPEN GAZETTE No. 55-17589. The expansion of the volume of the pressure chamber cancaves the ink interface (meniscus) inward the nozzle and causes a smaller ink droplet to be spouted from the nozzle under pressure, thereby reducing the diameter of the recorded dots.
The smaller diameter of the recorded dots makes the granularity substantially inconspicuous in a low density area and enhances the recording quality, but significantly lowers the recording rate. By way of example, where using only the smaller diameter dots having a dot diameter about half the standard dot diameter requires approximately four times the recording time, compared with using dots of a standard dot diameter. The possible measures to prevent the lowered recording rate increase the driving frequency for spouting ink droplets by four times and increase the number of nozzles by four times. Neither of these measures is, however, easily actualized.
One proposed technique causes ink droplets of different weights to be spouted from one identical nozzle and enables the tone expression (see, for example, the specification of the U.S. Pat. No. 5,285,215). This technique generates a plurality of identical pulse signals in one recording period and thereby makes a plurality of minute ink droplets. Before reaching the recording paper, the plurality of minute ink droplets join to produce a large ink droplet.
This proposed technique controls the spout of minute ink droplets and the spout of a large ink droplet obtained by uniting the plurality of minute ink droplets. A lot of conditions, such as the distance between the nozzle end and the recording paper and the relationship between the spouting rate of the ink droplets and the driving rate of the head, must be fulfilled to securely unite the plurality of ink droplets having substantially the same size before the ink droplets reach the recording paper. Another disadvantage is a narrow variable range of the recorded dot diameter.
The object of the present invention is thus to solve the above problems and provide an apparatus for and a method of driving an ink jet record head that widens the variable range of the recorded dot diameter, as well as a printing apparatus with this recording head driving apparatus.